On the first day of the crisis, Oct. 16, 1962, McNamara was among the few advisers who cautioned against attacking the Soviet missile sites before first thinking through the consequences. He advocated instead a naval blockade of Cuba—which became the United States' first counter-move—at least as a way to buy time. However, on Oct. 18, he switched and endorsed the Joint Chiefs of Staff's plan for an air raid followed by an invasion. "In other words," the tapes reveal him saying, "we consider nothing short of a full invasion as practicable military action, and this only on the assumption that we're operating against a force that does not possess operational nuclear weapons" (in other words, as long as the attack is launched before the Soviets had loaded warheads on the missiles). By Oct. 25, he has given up on the blockade entirely, saying, "I don't see any way to get those weapons out of Cuba—never have thought we would get them out of Cuba—without the application of substantial force … economic force and military force." On Oct. 27, McNamara urges Kennedy not to accept Khrushchev's offer of a missile trade and recites a series of steps that need to be taken "before we attack Cuba." The attach plan he endorses calls for massive airstrikes against the missile sites and bomber bases—500 sorties a day for seven days—followed by a land invasion.

Tapes of the ExComm sessions are available from the John F. Kennedy Library in Dorchester, Mass. The best analysis of the tapes, as well as the definitive history of the crisis, is Sheldon M. Stern's Averting "The Final Failure": John F. Kennedy and the Secret Cuban Missile Crisis Meetings (2003).